Ball screw

ABSTRACT

The invention provides a ball screw which is little subject to deterioration of operating characteristics and occurrence of wear, torque change, noise, vibration, etc. during operation due to stagnation or competition of balls. A ball screw comprising a screw shaft having a helical thread groove formed on the outer surface thereof and a nut having a thread groove formed on the inner surface thereof corresponding to the thread groove on the screw shaft which engages with the screw shaft with the interposition of a number of balls rollably provided in a helical ball rolling path formed between the two thread grooves, wherein the variation of the total of the gap between said balls during the rolling of the balls is smaller than {fraction (1/20)} of the diameter of the ball.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a ball screw. More particularly,the invention relates to a ball screw which is little subject todeterioration of operating characteristics and occurrence of wear,torque change, noise, vibration, etc. due to stagnation or competitionof balls.

[0002] A ball screw comprises a screw shaft and a nut. An outer surfaceof the screw shaft and an inner surface of the nut are provided with therespective helical thread grooves corresponding to and confronting witheach other. The helical space formed between the two thread grooves,i.e., ball rolling path is filled with a number of balls while allowingthe balls to rotate. In this arrangement, the screw shaft and the nutmake a relative helical movement with the rolling of these balls to makea relative movement in the axial direction.

[0003] Such a ball screw has a gap formed between the adjacent balls. Asthe balls roll, the total of all the gaps in one circuit varies withevery movement of one ball.

[0004] In particular, a ball screw for use in injection molding machineor the like, i.e., ball screw comprising balls having a diameterrelatively greater than the diameter of the shaft and hence having arelatively great scooping angle tends to have a great variation of thetotal of gaps (value obtained by totaling the gaps between the ballsover the entire ball screw).

[0005] Thus, a ball screw which varies greatly in the total of gapsbetween balls is subject to deterioration of operating characteristicsand occurrence of wear, torque change, noise (noise of ball slipping),vibration, etc. during the rolling of balls (during the operation ofball screw) due to stagnation or competition of balls to disadvantage.

SUMMARY OF THE INVENTION

[0006] It is therefore an object of the invention to provide a ballscrew which is little subject to deterioration of operatingcharacteristics and occurrence of wear, torque change, noise, vibration,etc. during operation due to stagnation or competition of balls.

[0007] The foregoing object of the invention will become apparent fromthe following detailed description and examples.

[0008] The foregoing object of the invention can be accomplished withthe following constitution. A ball screw, according to the presentinvention, comprises a screw shaft, a nut and a number of balls. Thescrew shaft has a helical thread groove formed on the outer surfacethereof. The nut has a thread groove formed on the inner surface thereofcorresponding to the thread groove on the screw shaft which engages withthe screw shaft with the interposition of the balls rollably provided ina helical ball rolling path formed between the two thread grooves. Inthe ball screw, the variation of the total of the gaps between the ballsduring the rolling of the balls is smaller than {fraction (1/20)} of thediameter of the ball.

[0009] In this arrangement, the ball screw is little subject todeterioration of operating characteristics and occurrence of wear,torque change, noise, vibration, etc. during operation due to stagnationor competition of balls. The ball screw is also little subject to wearor damage on the constituent members such as ball.

[0010] The invention can be applied also to a ball screw comprising aspacer for retaining a ball provided interposed between balls. In thiscase, the variation of the total of the gap between the spacers and theballs during the rolling of the balls is predetermined to be smallerthan {fraction (1/20)} of the diameter of the ball.

[0011] Such a ball screw can exert an effect of preventing the spacerfrom falling down in the ball rolling path or an effect of preventingthe spacer from being damaged besides the foregoing effect.

BRIEF DESCRIPTION OF THE DRAWING

[0012] By way of example and to make the description more clear,reference is made to the accompanying drawing in which:

[0013]FIG. 1 is a sectional view illustrating an embodiment of the ballscrew according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0014] An embodiment of the ball screw according to the invention willbe described hereinafter in connection with the sectional view of FIG.1.

[0015] A ball screw 1 comprises a screw shaft 2, a cylindrical nut 3,and a number of balls 4. The screw shaft 2 has a helical thread groove 2a having an arc section formed on the outer surface thereof. Thecylindrical nut 3 has a helical thread groove 3 a having an arc sectioncorresponding to the thread groove 2 a on the screw shaft 2 formed onthe inner surface thereof which engages with the screw shaft 2. Thenumber of balls 4 which are rollably provided in a helical ball rollingpath 6 having a substantially circular section formed between the threadgroove 2 a on the screw shaft 2 and the thread groove 3 a on the nut 3.

[0016] The nut 3 comprises a U-shaped ball circulating path 7 throughwhich the balls 4 circulate from the ball rolling path 6 to the ballrolling path 6. In some detail, the balls which are rolling and movingthrough the ball rolling path 6 until they revolve round the screw shaft2 along the thread groove 2 a by a plurality of turns, and then taken upby the ball circulating path 7 at one end thereof from which they movethrough the ball circulating path 7 back to the ball rolling path 6 atthe other. This operation is then repeated.

[0017] The nut 3 which engages with the screw shaft 2 with theinterposition of the balls 4 and the screw shaft 2 make a relativehelical movement with the rolling of the balls 4 to make a relativemovement in the axial direction.

[0018] The sectional shape of the thread grooves 2 a and 3 a may be aGothic arch, i.e., substantially V formed by combining two arches havingdifferent centers of curvature.

[0019] The balls 4 which are rolling and moving through the ball rollingpath 6 and the ball circulating path 7 have a gap C formed therebetween,as shown in FIG. 1. The total of the gaps C varies with the rolling ofthe balls 4. The variation of the total of the gaps C occurs due to thechange of the number of the balls 4 present in the respective sectionsof path.

[0020] The variation of the total of the gaps C can be adjusted bydesigned value of the height h of the ball circulating path 7 from thecenter of the screw shaft 2, the scooping angle γ of the ball 4 and theradius R of curvature of the bent portion of the ball circulating path 7(see FIG. 1). The adjustment of the variation of the total of the gaps Ccan be accomplished by predetermining the inner diameter of the ballcirculating path 7 so that the balls 4 are arranged with a staggeredmanner in the ball circulating path 7.

[0021] The invention will be further described in the followingexamples, but the invention should not be construed as being limitedthereto.

[0022] The ball screw 1 was evaluated for the variation of the total ofgaps C between the balls 4 in a row of circuit (Balls were arrangedgeometrically in a row of circuit on a computer. The total of gaps C wasthen calculated. The arrangement of these balls was then changed. Thetotal of gaps C was then calculated. This operation was repeated todetermine the variation of the total of gaps C.). The results ofevaluation on the ball screw 1 was then compared with that of aconventional ball screw.

[0023] The various dimensions and the variation of the total of gaps Cof Example (ball screw 1) and the conventional ball screw are shown inFIG. 1. TABLE 1 Example Conventional h (mm) 93.9 94.9 γ (°) 30 36 R (mm)40.9 35.9 Variation of total 0.32 1.17 gap (mm)

[0024] The ball screw of Example and the conventional ball screw are thesame in dimensions other than h, γ and R as follows:

[0025] * Diameter of screw shaft: 120 mm

[0026] * Ball center diameter (BCD): 123 mm

[0027] * Lead: 20 mm

[0028] * Diameter of ball: 15.875 mm (⅝ inch.)

[0029] * Number of circuits: 2.5 turns×4 rows

[0030] As can be seen in Table 1, the ball screw 1 of Example, h, γ andR of which are designed as set forth in Table 1, showed a variation oftotal gap as small as 0.32 mm, which is smaller than {fraction (1/20)}of the diameter of ball (approx. 0.79 mm). As a result, the ball screw 1of Example was little subject to deterioration of operatingcharacteristics and occurrence of wear, torque change, noise, vibration,etc. during operation due to stagnation or competition of balls.

[0031] On the contrary, the conventional ball screw showed a variationof total gap as great as 1.17 mm and thus was subject to deteriorationof operating characteristics and occurrence of wear, torque change,noise, vibration, etc. during operation due to stagnation or competitionof balls.

[0032] The present embodiment merely illustrates an example of theinvention and doesn't restrict the invention.

[0033] For example, the various dimensions of the ball screw 1 and theshape of the ball circulating path 7 may be properly modified so far asthe object of the invention cannot be impaired (For example, in order toadjust the variation of the total of gaps C, the inner diameter of theball circulating path 7 may be predetermined to be greater than thediameter of the ball 4 so that the balls 4 are arranged with a staggeredmanner in the ball circulating path 7). The ball circulating system isnot limited to the return tube system as used in the present embodimentbut may be an end cap system, a circulating top system, a guide platesystem or the like. Further, the invention can be applied to a ballscrew comprising a spacer for retaining a ball provided interposedbetween the balls.

[0034] As mentioned above, the ball screw according to the invention islittle subject to deterioration of operating characteristics andoccurrence of wear, torque change, noise, vibration, etc. duringoperation due to stagnation or competition of balls.

[0035] While the invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

What is claimed is:
 1. A ball screw comprising: a number of balls; ascrew shaft having a helical thread groove formed on an outer surfacethereof; a nut having a thread groove formed on an inner surface thereofcorresponding to the thread groove on said screw shaft which engageswith said screw shaft with the interposition of the balls rollablyprovided in a helical ball rolling path formed between said two threadgrooves, wherein a variation of the total of gaps between the ballsduring the rolling of said balls is smaller than {fraction (1/20)} of anouter diameter of the ball.
 2. A ball screw comprising: a number ofballs; a screw shaft having a helical thread groove formed on an outersurface thereof; a nut having a thread groove formed on an inner surfacethereof corresponding to the thread groove on said screw shaft whichengages with said screw shaft with the interposition of the number ofballs rollably provided with a spacer for retaining a ball interposedtherebetween in a helical ball rolling path formed between said twothread grooves, wherein a variation of the total of gaps between saidballs and said spacers during the rolling of said balls is smaller than{fraction (1/20)} of the diameter of said ball.
 3. The ball screwaccording to claim 1, wherein the variation of the total of gaps isadjusted by at least one of designed values of a height of the ballcirculating path from the center of the screw shaft, a scooping angle ofthe ball, and a radius of curvature of a bent portion of the ballcirculating path.
 4. The ball screw according to claim 2, wherein thevariation of the total of gaps is adjusted by at least one of designedvalues of a height of the ball circulating path from the center of thescrew shaft, a scooping angle of the ball, and a radius of curvature ofa bent portion of the ball circulating path.
 5. The ball screw accordingto claim 1, wherein said nut is provided with a ball circulating paththat has an inner diameter slightly larger than the outer diameter ofthe ball.
 6. The ball screw according to claim 2, wherein said nut isprovided with a ball circulating path that has an inner diameterslightly larger than the outer diameter of the ball.
 7. The ball screwaccording to claim 5, wherein said ball circulating path comprises areturn tube.
 8. The ball screw according to claim 6, wherein said ballcirculating path comprises a return tube.